1. Field of the Invention
The present invention relates to a method for erasing bright burn that occurs in a radiographic imaging device. The present invention also relates to a radiographic imaging device having the function of erasing such bright burn. More specifically, the present invention relates a method for erasing bright burn using specific visible light when the bright burn occurs in a radiographic imaging device, and to a radiographic imaging device having the function of erasing bright burn by means of such visible light.
2. Description of the Related Art
Conventional radiographic images such as X-ray images are widely used for diagnosis of diseases in medical practice. In particular, radiographic images produced with screen-film systems have been improved to have higher sensitivity and quality over a long period of history, so that they are still used as high-reliability, high-cost-performance imaging systems in medical practice around the world. However, the information of these images is what is called analog image information, which cannot be freely subjected to image processing or instantly transmitted electronically, in contrast to digital image information, which continues to progress in recent years.
In recent years, therefore, digital radiographic detectors are appearing such as computed radiography (CR) systems and flat panel radiography detectors (flat panel detectors (FPDs)). Using these devices, digital radiographic images are directly obtained, which can be directly displayed on a cathode ray tube or an image display such as a liquid crystal panel. Such images do not always have to be formed on photographic films. Therefore, such X-ray image detectors reduce the need to form silver-halide photographic images and significantly improve the convenience of diagnosis in hospitals and clinics.
Computed radiography (CR) is now accepted as a digital X-ray imaging technique in medical practice. However, computed radiography (CR) does not have enough sharpness or special resolution, and its image quality does not reach the level of the screen-film systems. Newly developed digital X-ray imaging technologies include flat panel detectors (FPDs) using thin film transistors (TFTs), such as those described in John Rowlands, “Amorphous Semiconductor Usher in Digital X-ray Imaging”, Nov. 1997, Physics Today, p. 24, and L. E. Antonuk, “Development of a High Resolution, Active Matrix, Flat-Panel Imager with Enhanced Fill Factor”, SPIE Vol. 3032, p. 2 (1997).
Besides such medical applications, radiographic imaging technology is also used in non-destructive testing. Non-destructive testing is a technique to detect harmful flaws in mechanical parts or structures without destroying the objects. In such non-destructive testing, screen-film systems and X-ray irradiation of objects have also been used to form radiographic images for use in detecting flaws in the objects. However, such testing has a problem in that the resulting images are analog and cannot be freely subjected to image processing or instantly transmitted electronically, in contrast to digital image information. In recent years, therefore, digital radiographic detectors such as computed radiography (CR) systems and flat panel radiography detectors (flat panel detectors (FPDs)) have been increasingly used.
John Rowlands, “Amorphous Semiconductor Usher in Digital X-ray Imaging,” Nov. 1997, Physics Today, p. 24, and L. E. Antonuk, “Development of a High Resolution, Active Matrix, Flat-Panel Imager with Enhanced Fill Factor,” SPIE Vol. 3032, p. 2 (1997) show the use of cesium iodide (CsI) as an unit for producing visible light from X-rays. Cesium iodide (CsI) converts X-rays to visible light with relatively high efficiency. Therefore, a flat panel using cesium iodide (CsI) in combination with thin film transistors (TFTs) attracts attention as a high-sensitivity X-ray image visualizing system.
Unfortunately, when a large amount of X-rays are applied to a CsI scintillator, a temporary increase in sensitivity (bright burn effect) occurs in the CsI scintillator, so that an artifact or afterglow appears in FPD images. The increased sensitivity returns to the original level as time elapses. However, this problem is difficult to address using FPD gain correction (sensitivity correction) every time imaging is performed. In addition, the sensitivity increase occurs non-uniformly in the image region and can lead to a problem such as a reduction in contrast or image degradation. Therefore, improvements have been demanded.
To overcome such a bright burn effect, JP 2003-107163 A discloses a method of heating a scintillator to prevent a bright burn effect.
However, when such heating is used to erase the generated bright burn effect, it takes a long time to heat and cool the scintillator. Therefore, this method is not suitable for applications where a scintillator is repeatedly used for short periods of time.
JP 4790863 B1 discloses that ultraviolet light is applied to a scintillator through a large number of small holes formed in a reflecting plate so that the scintillator can be refreshed and thus a bright burn effect can be prevented.
However, such application of ultraviolet light to a scintillator causes a new problem such as a reduction in the sensitivity of the scintillator itself although bright burn might be erased by the application of ultraviolet light. First of all, JP 4790863 B1 never discloses any data showing whether or not bright burn is effectively erased, and does not demonstrate that the erasing effect is really achieved.
Thus, there has been proposed no effective method to prevent bright burn in a scintillator.
JP 2012-28617 A discloses a radiographic imaging device including a photoelectric transducer for converting incident X-rays to light and generating charges in response to the light, a transistor for outputting a detection signal generated in the photoelectric transducer, and a converter for converting the X-rays incident on the transistor to ultraviolet light (10 to 400 nm in wavelength).
The converter (ultraviolet scintillator) converts X-rays incident on the radiographic imaging device to ultraviolet light and applies the ultraviolet light to the transistor (TFT) to refresh the charges remaining in the TFT, so that the TFT threshold is reset to the normal value, which makes it possible to obtain an output signal from the TFT under constant conditions.
Therefore, the device is characterized in that the TFT threshold can be reset to the normal value by applying ultraviolet light to the TFT. However, JP 2012-28617 A discloses nothing about what effect the ultraviolet light will have on a scintillator when the ultraviolet light is applied to the scintillator converting the incident X-rays to visible light.
As a result of studies, the inventors have found that in some cases, when ultraviolet light is applied to a scintillator converting X-rays to visible light, the sensitivity of the scintillator decreases.
L. Trefilova, B. Grinyov, L. Kovaleva, N. Kosinov, O. Shpylynska, “Transformation of defects arising in CsI(Tl) crystals under daylight”, Phys. Stat. Sol., 2(1), p 101-(2005) shows in FIG. 2 that a long-lasting afterglow appears when mercury lamp beams containing ultraviolet components (253.7 nm and 365.0 nm) are applied to cesium iodide crystals with no filter. Specifically, in the drawing where ln(t) represents the natural logarithm of emission attenuation time (seconds) and ln(I) represents the natural logarithm of emission intensity, the light emission continues even after ln(t)=6, namely, t=403 seconds, which is considered to show the presence of afterglow. Like the bright burn effect, the afterglow is a type of delayed light emission and will be a cause of contrast-reduction-induced degradation of images. Therefore, the drawing is considered to show unfavorable results.
M. A. H. Chowdhury, D. C. Imrie, “Thermal annealing and optical darkening effects in CsI(Tl) crystals”, Nucl. Inst. Methods Phys. Res. A, 432, p 138-(1999) states in Introduction that defects produced by coloration induced by ultraviolet light as well as gamma-rays can move in a high-temperature environment, which suggests that ultraviolet irradiation can cause the coloration of crystals and the formation of defects. It is well known that the presence of defects has an effect on delayed light emission, for example, from V. Babin, K. Kalder, A. Krasnikov, S. Zazubovich, “Luminescence and defects creation under photoexcitation of CsI:Tl crystals in Tl+-related absorption bands”, J. Luminescence, 96, p 75-(2002). Therefore, delayed light emission is also considered to cause a reduction in contrast and degradation of image quality.
In order to suppress the degradation of image quality, therefore, the application of ultraviolet light to a scintillator is not preferred, and rather light with a wavelength longer than that of ultraviolet light (or with energy smaller than that of ultraviolet light) should be applied.
WO2013/002327A discloses that the order of a series of imaging steps is so determined that imaging can be performed while avoiding the site where a bright burn effect occurs. This does not make any alterations to the bright burn effect and merely uses a specific order of imaging steps to avoid the bright burn-induced degradation of images.